Networks enable computers and other devices to communicate. For example, networks can carry data representing video, audio, e-mail, and so forth. Typically, data sent across a network is divided into smaller messages known as packets. By analogy, a packet is much like an envelope you drop in a mailbox. A packet typically includes “payload” and a “header”. The packet's “payload” is analogous to the letter inside the envelope. The packet's “header” is much like the information written on the envelope itself. The header can include information to help network devices handle the packet appropriately. For example, the header can include an address that identifies the packet's destination. Much like one envelope stuffed inside another, one or more packets may be stored within another packet. This is known as “encapsulation”.
Some packet processing systems use programmable devices known as network processors. Network processors enable software programmers to quickly reprogram network processor operations, for example, to adapt to changing protocols or provide new features. Some network processors feature multiple processing engines to share packet processing duties. For instance, while one engine determines how to forward one packet further toward its destination, a different engine determines how to forward another. This enables the network processors to achieve high packet processing speeds while remaining programmable.